1. Field of the Invention
The present invention relates to a multiple antenna (i.e., Multiple Input Multiple Output (MIMO)) system. More particularly, the present invention relates to an apparatus and method for mitigating inter-cell interference in the MIMO system.
2. Description of the Related Art
Multiple Input Multiple Output (MIMO) systems divide a transmit beam pattern forming scheme into a closed loop scheme and an open loop scheme.
The closed loop scheme is a scheme in which a Base Station (BS) transmits a signal in a transmission pattern determined considering downlink channel information provided from a Mobile Station (MS). The open loop scheme is a scheme in which a BS transmits a signal in a fixed beam pattern irrespective of a channel state with an MS. For example, the closed loop scheme includes a codebook based communication scheme.
In the case of the codebook based communication scheme, an MS selects a code vector or code matrix suitable to a channel with a serving BS, from a previously arranged and defined codebook, and transmits the selected code vector or code matrix to the serving BS. The serving BS transmits a signal using the code vector or code matrix provided from the MS. That is, the serving BS can transmit a signal using the code vector or code matrix suitable to the channel with the MS, thus maximizing a throughput.
In the case of using the codebook based communication scheme, a BS determines a transmission pattern considering only a channel with an MS. That is, because the BS considers no inter-cell interference, performance may be deteriorated due to neighboring cell interference.
Accordingly, a technology for controlling inter-cell interference in a wireless communication system has been proposed. For example, to control the inter-cell interference, the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) standard proposes a Coordinated Multiple Point Transmission/Reception (CoMP) technology. For another example, to control the inter-cell interference, the Institute Electrical and Electronics Engineers (IEEE) 802.16 standard proposes a Multi-BS MIMO technology.
The CoMP technology is divided into a Coordinated Beamforming (CB) scheme and a Joint Processing/transmission (JP) scheme. Here, the CB scheme represents a scheme in which one BS transmits traffic data for one MS, and control traffic is shared between neighboring cells. The JP scheme is a scheme in which at least two BSs transmit traffic data for one MS. For example, the CB scheme includes a Precoding Matrix Index (PMI) coordination scheme.
In the case of using the PMI coordination scheme, an MS feeds the worst PMI of great interference from a neighboring BS or the best PMI of less interference, back to a serving BS. Based on the feedback information of the MS, the serving BS makes PMI coordination requests to the neighboring BSs that exert interference on the MS. Based on information provided from the serving BS, the neighboring BS restricts the use of the worst PMI (i.e., performs PMI restriction) or recommends the use of the best PMI (i.e., performs PMI recommendation), thus being capable of mitigating inter-cell interference.
However, the PMI coordination scheme has several inherent problems.
Firstly, in the case of MSs feeding information back to a serving BS or communicating between BSs, the PMI coordination scheme uses a restricted bandwidth. Accordingly, because the PMI coordination scheme cannot perfectly remove all PMIs on all subbands giving interference, it is difficult to achieve ideal throughput owing to residual interference.
Secondly, in a case where cell boundary MSs send too many PMI coordination requests to interfering BSs so as to increase their own throughputs, there is a problem that the interfering BSs making the PMI coordination requests from the MSs suffer a shortage of bands and PMIs to provide service to their own MSs.
Lastly, in a case where cell boundary MSs send too few PMI coordination requests, there is a problem of increasing a probability that an interfering cell uses a band and PMI exerting interference on the MS that has sent the PMI coordination request.
Accordingly, there is a need for a way to efficiently reduce inter-cell interference while reducing a collision between cells.